Spatial light modulators (SLM or SLMs), as the term is used herein, are optical masks having one or more small picture element, PEL or pixel areas that are individually and selectively switchable by the operation of one or more writing light beams. SLMs that have been selectively written in this manner (i.e. data has been stored therein) are then used to modulate a reading optical wavefront, causing the reading wavefront to be either transmitted through the SLM (i.e. a transmission mode device), or causing the reading wavefront to be reflected from the SLM (i.e. a reflection mode device), the transmitted or reflected reading wavefront having a polarized pixel portions as is determined by the data stored in the modulator's corresponding pixel portions.
These optical masks are usually two-dimensional, and may comprise a plurality of small pixel areas that may be arranged in a two or a three dimensional matrix of pixel rows and pixel columns.
In an optically addressable SLM, a write beam(s), for example a visible laser beam(s), programs or activates the individual pixels of the SLM to subsequently rotate the polarization, change the amplitude, or retard the phase of a read beam(s), for example an infrared laser beam(s). The write beam(s) programs the SLM by activating individual photosensitive pixel areas of the SLM. That is, those modulator areas that are to be programmed to rotate the polarization, change the amplitude or retard the phase of a read beam(s) are activated by a write beam(s).
U.S. Pat. No. 4,538,884 is an example of such an SLM. In the device of this patent, a pair of glass plates 1a and 2a support a pair of transparent electrodes 2a and 2b having a external source of voltage (not shown) applied thereto. A photoconductive layer 9, which can be amorphous silicon is supported on electrode layer 2b. A plurality of aluminum reflectors 8 are incorporated into a transparent insulating layer 7 and are supported on the surface of the photoconductive layer, with the reflectors directly adjacent to the photoconductive layer. An apertured shading layer 5 of carbon or metal is carried on the transparent insulating layer, each apertures 6 facing one of the reflectors. The space intermediate transparent insulating layer 7 and transparent electrode 2a is occupied by a liquid crystal 3.
The types of known liquid crystals include nematic liquid crystals, cholestic liquid crystals, smectic liquid crystals, and chiral smectic liquid crystals, of which electroclinic smectic A.sup.* and ferroelectric smectic C.sup.* are two examples.
A preferred liquid crystal material useful in the practice of the present invention, but without limitation thereto, is ferroelectric smectic C.sup.* or H liquid crystal material described in U.S. Pat. No. 4,367,924.
A preferred voltage/current generating light sensitive layer useful in the practice of the present invention, but without limitation thereto, is a hydrogenated amorphous silicon (a-Si:H) photovoltaic/photodiode layer.
The use of amorphous silicon photoconductor means in a liquid crystal SLM is suggested in the article "Amorphous silicon photoconductor in a liquid crystal spatial light modulator", by Paul R. Ashley and Jack H. Davis, APPLIED OPTICS, Jan. 15, 1987, Vol. 26, No.2, at pages 241-246. The device of this article uses an external bias voltage supply.
The use of amorphous silicon photoconductor means and ferroelectric liquid crystal means in a liquid crystal device is suggested in the article "High-speed light valve using an amorphous silicon photosensor and ferroelectric liquid crystals", by N. Takahashi, H. Asada, M. Miyahara and S. Kurita, APPLIED PHYSICS LETTERS, Vol. 51, No. 16, 19 Oct. 1987. Here again an external power supply is required.
The device of the present invention differs from prior art devices in that it is self-powered, i.e. no externally applied electrical power is required. Rather, the invention provides an internal photovoltaic/photodiode light sensitive layer and a liquid crystal layer that are sandwiched between two electrically conductive and light transparent layers. An electrical short circuit (i.e. a low impedance circuit) interconnects the two electrically conductive layers.
The use of a ferroelectric liquid crystal in an SLM having reflective mode photodiode or photoconductive amorphous silicon portions is suggested in THE PROCEEDINGS OF SPIE--THE INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING, Vol. 754, 13-15 Jan. 13-15 1987, at pages 207-212. In the photodiode embodiment of this article, an external voltage source is applied to the SLM to reverse bias the photodiode. When the photodiode is in the dark, the supply voltage is dropped across it, and the ferroelectric liquid crystal is maintained in its off state. When the photodiode is illuminated, the photodiode produces a current that charges the ferroelectric liquid crystal and switches the crystal to its on state.
The use of external solar cells to power liquid crystal devices is taught by the art. For example U.S. Pat. No. 4,475,031 describes a sun sensitive window, and U.S. Pat. No. 4,620,322 describes a welder's eyeshield, having liquid crystal elements wherein a solar cell that is external of the liquid crystal element is used to activate the device. Note that in each case, the solar cell is an external source of power, and is not an integral part of the light modulating device. That is, the incoming light wavefront does not both activate selected areas of the device, and at the same time cause an operating voltage/current to be generated.
These patents thus fail to teach the present invention wherein a self powered SLM, having no external power source is created by using a photovoltaic film whose selectively illuminated pixel area or areas generate the power that is required to switch only the corresponding pixel areas of the liquid crystal.